More specifically, said landing gear, which can be brought in the usual way into one of two positions, namely an up position and a down position, is of the type comprising at least one noise reducing means which is associated with at least one constituent element (tube, strut, attachment, pipe, etc) of said landing gear.
The present invention applies to all types of landing gear and, in particular, applies equally to nose gear and to main landing gear, whether this be under the wing and/or under the fuselage.
It is known that aerodynamic noise is a critical parameter in aeronautics and that each new aircraft has, in particular in order to gain certification, to comply with international regulations laid down by the ICAO (which is an English-language acronym for “International Civil Aviation Organization”), which is very strict on noise. In addition, the International Civil Aviation Organization will probably in years to come lower the permissible noise limits. What is more, certain airports have laid down noise-related restrictions that are even tighter than these international rules.
It is known that aircraft landing gear is a significant source of noise during takeoff and also, especially, during landing phases (the approach, intermediate and final phases). It is generally accepted that aircraft landing gear produces approximately 30% of the total noise generated by an aircraft during the landing procedure. In addition, measurements have demonstrated that the noise caused by landing gear is emitted over a broad range of frequencies ranging from low frequencies of the order of 90 Hz to frequencies in excess of 4 kHz. The noise emitted by landing gear is therefore not a low-frequency phenomenon and is very annoying particularly to populations living near airports.
This noise is created essentially by the airflow through the constituent elements of the landing gear when the landing gear is down, which causes turbulence phenomena which, by interacting with the structures, causes noise. This is in part due to the fact that not all the various constituent elements (tubes, struts, attachments, wheels, etc) of the landing gear are aerodynamic. In the context of the present invention it will be considered that an element that is aerodynamic is an element that has a shape that causes little disturbance to the airflow and therefore generates little air turbulence that is a source of noise. The stream of air that flows around these non-aerodynamic constituent elements therefore causes turbulence and generates noise. The noise is generally caused when these elements shed the turbulence. In addition, a broadband noise may also be generated, when free turbulence interacts with the surfaces of other elements located downstream. The overall noise produced by landing gear therefore arises chiefly out of turbulence, and its interaction with the elements of the gear, be it aerodynamic or otherwise.
One way of reducing this noise might be to surround some of the constituent elements of the landing gear, or even the entire landing gear, with rigid fairings that have an aerodynamic shape and allow the airflow to be kept away from the non-aerodynamic elements. However, a solution such as this leads to numerous disadvantages such as, in particular, an increase in the cost of manufacture, difficulties in inspecting the landing gear or some of its constituent elements, and an increase in weight.
Document WO-2004/039671 discloses noise reducing means which are arranged on landing gear. These noise reducing means comprise, in particular, a deflecting fairing the purpose of which is to alter the direction of the airflow so as to keep it away from noisy constituent elements of the landing gear with which this fairing is associated. Keeping the air away from a noise-generating zone allows the overall noise to be reduced. This fairing may be equipped with a perforated zone the purpose of which is to limit the amount of air diverted and the potential negative effects thereof. In addition, the (undiverted) air which passes through this perforated zone is slowed so that the resulting noise is also lowered. This earlier document makes provision for under half the surface area of this perforated zone to actually be perforated.